Metal halide lamps have an inner arc tube containing a fill of an arc-sustaining material and surrounded by an outer glass envelope. The metal halide lamp's arc tube fill includes a rare gas for starting and a quantity of mercury. However, as compared to a mercury lamp, the metal halide lamp's emission spectrum is primarily due to the presence in the arc tube fill of one or more metal halides, usually iodides. These metal halides are responsible for a much higher luminous efficacy and better color rendering capability of the lamp output than is possible for the mercury vapor lamp.
The luminous efficacy, color rendering index and other lamp output characteristics may be varied, depending upon the particular composition of the metal halides in the arc tube. GTE's Metalarc M100/U lamp, with a NaIScI.sub.3 CsI chemistry, has a CRI (color rendering index) of 65, an initial LPW (lumens per Watt) of 85, and a 10,000 hour lifetime. In the lighting industry, these specifications are considered very good for standard lighting applications. Each chemical in the lamp is chosen to contribute specific effects to the lamp's performance. The mercury controls the current-voltage characteristics of the lamp, and the alkali metal iodides adjust the color quality, and contribute to lumen output through strong emissions. Scandium is added to the lamp as an iodide and as a pure metal. The scandium iodide improves color quality by adding a variety of lines to the color spectrum. The elemental scandium chip is used to adjust the metal/iodine ratio in the lamp and to getter oxygen impurities.
By modifying the above chemistry by the replacement of the element Cesium with Lithium to form a chemistry of NaIScI.sub.3 LiI, the resulting lamp has an improved CRI of 73 and a high LPW of 85 while still maintaining the 10,000 hour life.
In general, maintaining a proper arc cold spot temperature for the arc tube is conducive to long lamp life. The cold spot temperature is dependent on multiple factors such as light transmissive properties, diameter, length, and wall thickness of the arc tube. Providing an evacuated outer jacket tends to increase the cold spot temperature. The presence of gases in the outer jacket tend to decrease the wall temperature due to convection. Hence, in the vacuum outer jacket of lower wattage bulbs with their smaller volume, it is important to control the presence of gas.
Even though the outer jacket is evacuated, the presence of residual materials may tend to cause darkening of the outer envelope and reduce the lumen output of the lamp. The presence of gas in the outer envelop can result in lower cold spot temperatures which may result in poorer lamp performance. During the operation of the lamp, undesirable materials including hydrogen tend to outgas into the outer envelope so that it is desirable to maintain the vacuum integrity of the outer envelope throughout the entire life of the lamp.
Heretofore, getters have been utilized in the prior art to maintain the vacuum in the outer envelope. However, although prior art getters may be suitable for the higher temperatures achieved in the higher wattage lamps, such getters are not necessarily desirable for lower wattage lamps which operate at lower temperatures. Also, many prior art getters have the disadvantage that high activation temperatures are required to initiate the gettering properties. This activation may be performed prior to lamp operation as a separate step or may occur during operation of the lamp. In either case, proper activation of the getter is a concern. Hence, it is desirable to produce an improved low wattage lamp which obviates one or more disadvantages of prior art lamps. Especially, desirable is a low wattage lamp which is properly gettered so as to desirably enhance the performance of the above discussed lamps.